homer gps and GPS Antennae - How do they differ ?
For recreational and traveling purposes, standard GPS receiver antennas can be classified into two groups:
1. Upright antennas (or rather Quadrifilar helix antennas):
Rectangular in shape, mostly visible and external to the main housing of the receiver
Can detect satellites right on the horizon
Cannot normally detect satellites directly overhead
Should be held upright for best reception
2. Flat antennas (or rather Patch microstrip antennas)
Flat patch, usually not visible and internal to the receiver's housing
Can detect satellites directly overhead
Cannot detect satellites on the horizon
Should be held flat for best reception
External antennas linked via cable to the GPS receiver, are normally much more sensitive than internal antennas and allows for comfortable positioning within a vehicle, boat or other enclosures. Some ",active", external antennas are available to actively amplify the antennas signal before sending it to the GPS receiver in order to compensate for the signal loss through the cable.
How do I convert a location's waypoint to a different map datum?
The mathematics involved in map datum conversions are quite involved and will not be discussed here. Have a look here for software capable of transforming between different map datums. However, there is quite and easy way to convert from one map datum to another - available directly on your GPS !
Say for example that you would like to convert a waypoint from the CAPE map datum to the WGS84 map datum.
1. Set your GPS map datum to ",CAPE",.
Enter a waypoint such as
S 25d50.653m E028d09.282m.
2. Now set your GPS map datum to ",WGS84", and note the converted waypoint.
S 25d50.687m E028d09.264m
The differences in the longitude and latitudes in the above example, translates to an error of 69 meters if map datums are used incorrectly.
Selective availability (SA)
The GPS satellites are owned and controlled by the U.S. Department of Defence and they used to degrade the accuracy of the GPS signal available to commercial users. This is known as ",Selective Availability", (commonly known as ",SA",).
Since 01 May 2000, SA was however dropped by the Clinton Administration and we no longer have to worry about it. See the press release. (however, it is not to say that SA may not be switched on again for whatever reason !!!)
What reference is used for the waypoints of towns, cities and other large surface areas ?
Traditionally, post-offices was used by some institutions as a reference point for cities and towns. Due to the mapping of other large surface areas like dams, lakes and pans, the currently used method is to use the calculated centroid of the area as the reference point.
Seth Tyler writes for GPSYES.com where he presents articles and information on homer gps, gps equipment sales, and related topics...